The goal of this research program is to explore new laser technologies, new spectral windows, and three- photon fluorescence microscopy for imaging deep into scattering tissues, and then demonstrate the new methodologies in in vivo biological imaging. The proposed research consists of two sequential thrusts: the first involves the development of a novel energetic excitation source for the exploration of the new spectral window between 1600 and 1800 nm (i.e., the 1700-nm spectral window) with significantly reduced tissue scattering; the second thrust concentrates on demonstrating the new methodologies for in vivo deep tissue imaging based on three-photon excitation, improving the signal-to-background ratio by orders of magnitude and extending the depth penetration of multiphoton imaging. The proposed program is based on three major innovations: (1) 1700-nm spectral window for significantly reduced tissue scattering, (2) 3PE as a new excitation modality to simultaneously improve the SBR and extend the accessibility of fluorophores in deep tissue imaging, and (3) an excitation source tailored for in vivo deep tissue three-photon fluorescence microscopy at the 1700-nm spectral window by using soliton self-frequency shift in a photonic crystal rod to generate energetic, wavelength tunable solitons seeded from a fiber laser. We aim to demonstrate a new generation of multiphoton microscopic imaging tool that can reach an ultimate imaging depth of 3 mm or beyond within intact biological tissues such as the mouse or rat brain. The successful completion of this program will have a broad impact on a wide variety of biological and biomedical research fields where high-resolution imaging deep within intact tissue is required.